1. Field of the Invention
This invention relates generally to corona charging, and more particularly to an improved corona charger which is more compact and has a nonconductive shell for focusing a substantially constant current in a small area. The charger is particularly suitable for transfer charger operation in copiers.
2. Description of the Prior Art
Referring to FIG. 1, conventional corona charger designs for electrophotographic applications generally utilize a thin wire 10 surrounded by a grounded metal shell 12. Corona wire 10 is typically driven at a D.C. potential of say -5.4 kV, which results in a characteristic plate current-to-potential curve shown in FIG. 2.
Although the charger design of FIG. 1 is adequate for transfer purposes, its performance is not optimum. Consistent transfer performance requires that the electrostatic field generated in the transfer region be consistent. The transfer field is a function of the amount of charge deposited. The charger must lay down a constant level of charge irrespective of the receiver characteristics.
In conventional chargers with grounded shells, the current flow is divided between the grounded shell and the receiver surface, and the distribution is dependent upon the potential of the receiver surface. Tyically then, a more insulative receiver would charge to a higher surface potential than a more conductive receiver because the conductive receiver lets the charge migrate from its surface.
Conventional transfer chargers do not act as constant current devices. If the charger has a constant current power supply to the corona wire 10, the current is divided between the shell 12 and the receiver. As the potential of the receiver surface increases, the ratio of current to the shell and receiver changes. A charger set up to deliver the correct amount of charge for a receiver of one conductivity would not deliver the correct amount of charge if the receiver's conductivity changed because the charger is voltage sensitive.
For example, a dry paper receiver will be charged to a voltage of several hundred volts to deliver the desired transfer field (typically around 33 to 35 volts per micron). Under conditions of high humidity, paper is more conductive, and charge is conducted away from the receiver's surface. This lowers the potential of the surface, and the transfer charger responds to the lower surface potential of the receiver by tending to overcharge the receiver. An overcharged receiver creates image degradation due to breakdown, as well as complicating detack due to the excessive charge levels.
Another problem with conventional chargers is that, under dry conditions, they are not able to charge high enough due to the cutoff potential of the charger. As the receiver potential approaches this cut-off potential, corona output is suppressed and current output to the receiver goes to zero. This could be overcome just by increasing the wire potential, but current output will get excessively high, and high potentials result in overcharging in more humid conditions.
The problems mentioned above can be minimized by designing the charger to operate in a mode that better aproximates constant current operation and by reducing the transfer time. It is known that transfer to transparency material can be improved by increasing the charger cut-off potential. Unfortunately in conventional designs, this involves higher wire currents which generates excessive amounts of ozone. Wire to receiver arcing also becomes a danger due to the higher wire potentials involved. Since the speed of the receiver as it passes under the charger is fixed, typically due to other machine constraints, transfer time can only be reduced by decreasing the charger width. However, reduction of the charger width is not a possibility in conventional charger designs. Because of the high potential placed on the wire, a minimum distance d between the wire and conductive surfaces must be maintained to prevent wire to shell arcing. Accordingly, the charger must have a cross sectional dimension in the direction of travel of the receiver of at least twice the minimum distance d between the wire and conductive surfaces.